1. Field of the Invention
The present invention relates to a clocking system for a multichannel data transmission system which may include a temporary storage medium, such as magnetic tape. More particularly, the invention relates to an improved clocking system which utilizes a single clock for the multiple channels, rather than a separate phase locked loop clock regeneration circuit for each channel. The single clock signal operates at a frequency F that is an integral multiple of the channel baud rate. Means are provided to continually monitor the phase error of each channel, and a correction signal is derived and applied to the clock signal which is proportional to the average of the phase errors of the individual channels.
2. Description of the Prior Art
In self-clocking data transmission systems which may include temporary storage medium, the function of the read electronics is to extract both data and clock signals from the read signal. Such multichannel, high-density recording systems have heretofore used a separate phase locked loop for each track to extract the necessary timing information from the read signal. This results in fairly complex clock the detect circuitry that represents a significant cost with respect to such systems.
U.S. Pat. No. 3,452,294 to Mumford, et al. describes a system providing rate synchronization of a network of digital stations. In the system, each station has its own oscillator and the oscillators are maintained in synchronism with each other by comparing the oscillator at a given station repeatedly with the oscillators at each of the other stations. A unit is added to the compared oscillator signal for each other oscillator found to have a higher frequency and a unit is subtracted for each oscillator which is found to have a lower frequency. Accordingly, the system attains some measure of synchronism.
U.S. Pat. No. 3,128,430 describes a phase shifting system for phased antenna arrays. However, the patent fails to disclose any means to correct or modify the phase relationships between the various arrays.
U.S. Pat. No. 4,013,962 describes a receiver selecting voting system in which the lowset noise audio signal produced by a number of different radio receivers is selected and coupled to a monitoring speaker. Each receiver obtains signals from a transmitter and produces either corresponding audio signals or squelch indicating status tones which are transmitted to a voting control center. The voting control center selects the unsquelched receiver audio signal having the lowest noise level and couples the signal to a monitoring speaker.
None of these patents discloses a suitable technique for replacing a multiplicity of clock signals with a single clock signal which is corrected to reflect the average phase displacement of multiple channels whereby digital data can be suitably recovered.